1. Field of the Invention
The invention relates to a display device for displaying a video signal on a display screen of a display tube comprising at least one control electrode and deflection coils for deflecting at least one electron beam current, said display device further having an input for receiving the video signal, means for determining a derivative of a luminance component of the video signal, a scan velocity modulator for modulating the deflection rate of the electron beam current in the display tube in dependence upon the determined derivative, a position error correction circuit for correcting the video signal in dependence upon the derivative of the luminance component of the video signal, and means for applying the corrected video signal to the control electrode of the display tube.
2. Description of the Related Art
A display device of this type is known from U.S. Pat. No. 4,183,064. In this known display device, the position error is corrected by enlarging the portion of a display line having a higher luminance with respect to a portion having a smaller luminance and by subsequently applying scan velocity modulation (so as to obtain an improved definition) at which the dark/light transition is delayed and the light/dark transition is brought forward. As a result, the picture to be displayed is displayed with the original picture contents (the same quantity of light and dark portions as in the original video signal). In this solution, a second error (enlarging the light portions) is deliberately introduced to correct the first error (reducing the light portions as a result of scan velocity modulation). This is not an ideal solution because the two errors must compensate each other in this case. Correcting a non-linear error by means of a linear system is not very well possible. The drawback is that the position error cannot be satisfactorily corrected in this way. At a less sharp transition from light to dark (or conversely), the second error will be too large so that it will overcompensate the first error, whereas with a very sharp transition, the second error is too small so that the first error is not fully compensated. A further drawback is that it is not easy to enlarge the portions of the video signal having a higher luminance/brightness. Moreover, by enlarging the light portion, the beam current is increased so that the definition is adversely influenced due to spot growth.
To give pictures a better (impression of) sharpness, manufacturers focus on improvements of the display tube, inter alia by providing an improved phosphor layer and by improving the electron gun/guns. Moreover, scan velocity modulation of the electron beam deflection is used in a display tube (as is described, for example in the above-mentioned U.S. Patent). In this method the scan velocity (deflection rate) is adapted to the picture contents, notably to brightness variations. In scan velocity modulation, the derivative of the luminance component of the video signal is determined. Generally, the second derivative of the luminance component is used, which second derivative is applied to a voltage amplifier, an output of which applies a voltage to, for example, a scan velocity modulation coil. If a voltage-controlled current source is used instead of the voltage amplifier, the first derivative of the luminance component is taken. Actually, the scan velocity modulation coil is then the second differentiator. The scan velocity modulation is proportional to the second derivative of the voltage across the coil. By using scan velocity modulation, a position error is produced on the display screen (the video information rate is no longer synchronous with the scan velocity) at which a dark/light transition of the video signal is shifted to the right and a light/dark transition of the video signal is shifted to the left on the display screen. Consequently, portions of the video signal having a higher brightness/luminance are reduced with respect to portions of the video signal having a smaller light intensity. For example, when a plurality of successive squares (for example, a chessboard) is displayed, this effect can be clearly observed: larger (darker) and smaller (lighter) squares instead of squares all having the same size.